Aluminum is produced in Hall-Heroult cells by the electrolysis of alumina in molten cryolite, using conductive carbon electrodes as anodes. During the reaction the carbon anode is consumed at the rate of approximately 450 kg/mT of aluminum produced under the overall reaction ##STR1##
The problems caused by the consumption of anode carbon are related to the cost of the anode consumed in the above reaction and to the impurities introduced into the melt from the carbon source. The petroleum cokes used in manufacturing the anodes usually have significant quantities of impurities, principally sulfur, silicon, vanadium, titanium, iron and nickel. Sulfur is oxidized to its oxides, causing particularly troublesome workplace and environmental pollution. The metals, particularly vanadium, are undesirable as contaminants in the aluminum metal produced. Removal of excess quantities of the impurities requires extra and costly steps when high purity aluminum is to be produced.
If no carbon were consumed in the reduction the overall reaction would be 2Al.sub.2 O.sub.3 .fwdarw.4Al+30.sub.2 and the oxygen produced could theoretically be recovered, but more importantly no carbon would be consumed at the anode and no contamination of the atmosphere or the product would occur from the impurities present in the coke.
The aluminum industry has long sought to develop an inert ceramic anode to replace the consumable carbon anode used in Hall-Heroult electrolysis. In recent years the development effort has been accelerated and, although no instance has been reported where aluminum is produced commercially with inert anodes, significant strides have been made toward the realization of this goal. The cost of the anodes, due almost entirely to the cost of the materials from which they are made, exceeds by an appreciable amount that of baked carbon anodes now in use. However, the longer lifetime of a ceramic or cement anode, one to two years, results in a net savings in anode cost per unit of aluminum produced. From an operational viewpoint, the increased capital worth of a cell equipped with new inert anodes versus one equipped with less expensive carbon anodes justifies taking additional precautions to prevent the anodes from being damaged.
The inert anode materials disclosed to date all contain metal oxides as their principal constituent for the reason that oxides are stable to the oxygen anode product. However, most metal oxides are chemically reduced by liquid aluminum at high temperature forming Al.sub.2 O.sub.3 and a metal ion which, in a Hall-Heroult cell, is co-deposited with the aluminum ions at the cathode to contaminate the aluminum product. Attack of the anode by aluminum does not occur to any appreciable extent during normal electrolysis because the oxygen gas produced at the surface of the anode acts as a protective barrier to aluminum attack and additionally stabilizes the oxide-based anode material. It is during periods when electrolysis is interrupted, e.g., during an extended power failure, that the anodes are susceptible to chemical reduction and the aluminum metal in the cell to subsequent contamination. Power outages for periods as little as five minutes may be sufficient to produce these effects, the actual time being dependent on the electrode material and the operating anode-cathode spacing. In the worst case, when power cannot be restored for several hours, the cells are subject to freeze-up which would be catastrophic to the anodes resulting in a serious financial loss.
The invention described herein, an anode retraction device, is capable of sensing a power interruption and withdrawing the anodes from the melt to preclude or minimize such damage.